The present invention relates to a method for forming a multilayer semiconductor structure intended for electronics, optics or optoelectronics that includes a buried or intermediate layer. In particular, the method includes forming a protective layer between the intermediate layer and a surface layer, wherein the protective layer is made from a material chosen to be sufficiently resistant to a chemical treatment to protect the intermediate layer from chemical attack
In this document, a buried layer or intermediate layer means any layer included within a structure, which may include material above and below it, and having material properties significantly different from the properties of the adjacent material. Existing semiconductor structures for use with electronics, optics or optoelectronics frequently include an intermediate layer. For example, SOI (Silicon On Insulator) structures include a buried layer that is an electrically insulating layer which can be made of, for example, SiO2 or Si3N4.
Techniques such as epitaxy, bonding wafers, and transferring or detaching thin layers, are used to manufacture such semiconductor structures or wafers. These techniques are usually used after, at the same time as, and/or before a chemical treatment is used. Chemical treatments are used for various reasons, such as for cleaning the wafer or for chemically etching one or several layers. In some cases, these chemical treatments may affect the quality of the overall structure, particularly when one or several of the intermediate layers or buried layers are etched. By definition, it seems that a buried layer would not come into contact with the outside of the structure (and therefore does not have any access paths to chemical species), but this is not always the case for some structures. In particular, a buried layer can be exposed to come into contact with the outside portion of the structure at an edge of the structure. In addition, a buried layer may be subjacent to a thin layer, and the surface of the thin layer is in contact with the exterior and includes some defects, all of which can provide access paths to the buried layer for chemical etching species. Such defects may occur due to the nature of the material of the thin layer, or may be caused by contaminants, precipitates, and the like.
These intrinsic problems with etching buried layers within structures occur particularly in cases in which the structures are provided with a particularly thin layer on the surface and on the buried layer. Such thin layers are typically on the order of a few tens of nanometers. The presence of defects in these thin layers can possibly have dramatic consequences on the quality of the final structure after the chemical etching treatment occurs.
This problem occurs particularly in chemical treatment that use hydrofluoric acid HF. For example, SOI (Silicon On Insulator) structures typically include a particularly thin useful layer of silicon on an SiO2 layer, and a substrate typically made of silicon. HF-type etching is typically used on such structures for deoxidation, for example after an oxidizing heat treatment. If the thin silicon layer has defects like those mentioned above, the buried SiO2 layer could be etched by the HF solution. A defect of 100 nanometers or more in a silicon layer of this type can provide access to the buried oxide. Consequently, holes may be formed having a diameter of several microns or several tens of microns in the oxide layer, due to the action or chemical attack of the chemical species, and these holes are referred to as “decorations” (for example, see cavities 17 and 17′ in FIG. 2).
Therefore, defects in the useful layer, also called HF defects, can have a dramatic effect on the quality of the buried layer and on the entire structure. Those skilled in the art define these HF defects as being “killer” defects. Further, the presence of HF defects close to an SiO2 buried layer can cause other detrimental effects in the case of removable substrates.
Production of removable substrates by controlling the bonding energy are described particularly in International Application Publication No. WO 02084722. The removable substrates are produced by reducing the bonding forces between a substrate and a wafer (or a thin layer) that are bonded together, so that the bonding forces are less than the optimum bonding forces. A weakened zone is created in the substrate—wafer (or layer) assembly to form the removable substrate. But several problems can occur when etching the buried SiO2 by chemically treating (for example, by using an HF treatment) the removable substrate in which this type of weakened zone is located within a buried thickness of SiO2. In particular, a problem can occur if the bonding energy has not been controlled sufficiently optimally, for example, if it is too weak. This will cause edge delamination problems, wherein silicon grains close to the periphery of the layer are etched off (for example, see FIG. 5). In addition; if HF defects are present in the layer subjacent to the buried SiO2 thickness, then decorations are formed in the buried layers under the action of the HF chemical species. Due to bonding imperfections in the weakened zone, these decorations tend to become larger than they otherwise would have been if optimum bonding had occurred. Such decorations can be about 10 to about 100 times larger than in the case of a non-removable substrate, which may cause the thin layer above the decorations to loose its stiffness and tear. This can result in the presence of particles on the wafer and therefore pose contamination problems during later processes of the SOI structures, removable substrates or other structures (for example, see FIG. 7).
In an effort to overcome such chemical etching problems concerning buried layers, an attempt was made to improve the quality of the useful layers by reducing the defect density, to make it more difficult for chemical species to access the buried layer. Other techniques attempt to use chemical species that are less aggressive and thus that will not detrimentally affect the buried layer during the chemical treatment.